It is *technically* possible, but first they need to check on collisions of your username with other usernames (usernames need to be unique). And, depending on the forum capabilities, they might possibly even do a MySQL query for that.

Sorry for being late to the party, but.. this username is from 3 years ago, back then I didn't know I'd get really into The Walking Dead and want to use these forums- can I have mine changed to Heisenberg or Doublemint or something? Please?